The present invention relates to a tyre for a vehicle wheel in which at least one of the beads comprises a seat having a generatrix, the axially inner end of which lies on a circle of diameter greater than the diameter of the circle on which the axially outer end is located, sidewalls of unequal heights and specific anchoring zones for each of the sidewalls. This type of design is particularly suited to the new generations of tyres which can be used, within certain limits, in conditions of low pressure, or even zero or virtually zero pressure, with the risk of separation of the tyre from the rim on which it is fitted being reduced. This concept is frequently designated by the expression “extended mobility”.
For a long time, tyre manufacturers have been trying to develop a tyre which does not create any source of potential risk or danger in the event of an abnormal drop in, or even total loss of, pressure. One of the difficulties encountered relates to travelling with a flat tyre or at very low pressure. In fact, when travelling at very low pressure, or even at zero pressure, with conventional tyres, the beads are at great risk of separating from the periphery of the rim against which they were held by the pressure.
Numerous solutions have been tested in order to overcome these disadvantages. Frequently, these solutions cause additional difficulties in fitting and removing the tyre to/from the rim.
Furthermore, the tyre clamping function onto the rim is an essential function for ensuring the behaviour of the tyre in operation. In fact, this function bears directly or indirectly on numerous aspects such as fitting (sometimes known as “clipping”) or fastening of the tyre, the tightness of the tyre, rotation on the rim etc. These functions are all critical and require specific characteristics and rigorous manufacture of the products, in particular if high standards of quality are desired. Outside, the rims and tyres of the same code are often of slightly different dimensions, primarily due to manufacturing tolerances. These dimensional variations make it more difficult to comply with the various functions listed above.
Two major types of solution are used industrially to fulfil these functions. Firstly, for traditional tyres, it is the bead wire which performs all these functions simultaneously.
More recently, in various types of product manufactured by the Applicant, the conventional bead wire is replaced by an anchoring zone in particular comprising arrangements of circumferential cords which cooperate with the carcass-type reinforcement structure via an anchoring or bonding compound. Here too, the anchoring zone performs all the above-stated functions.
However, in both these cases, it is difficult to optimise certain parameters because, very often, improvement in one parameter causes another to deteriorate. There are thus certain limits to making such compromises between a gain on one hand and a loss on another, since it is often difficult to tolerate poorer performance for certain aspects.
EP 0 582 196 discloses a tyre comprising a tread extended by two sidewalls and two beads and also a carcass anchored in the two beads to an annular reinforcement. The carcass is formed of adjacently arranged cords which are aligned circumferentially and are in contact with at least one layer of bonding rubber of very high elasticity modulus in the hooking zone of the bead comprising the annular reinforcement. In this tyre, the annular reinforcement of the hooking zone of the bead is formed of stacks of circumferential cords with interposition of a layer of bonding rubber of very high elasticity modulus between the reinforcement cords of the carcass and these stacks. This embodiment is intended for tyres of conventional type, with the beads being held against the rim hook due to the inflation pressure of the tyre. In this type of arrangement, there is a predominance of stresses of the lateral or axial type, which induces major compressive forces which act substantially axially from the sidewalls towards the centre of said bead. These forces increase according to the inflation pressure. The increase in pressure tends to make the bead slide against the hook, radially towards the outside. The stresses induced radially towards the inside, against the seat of the rim, decrease with the increase in pressure, or with any increase in the tension of the carcass-type reinforcement structure.
It will furthermore be noted that the stacks of cords are aligned in a direction substantially parallel to the orientation of the profile of the rim hook against which the bead bears. The profile of the bead of this type of tyre is relatively narrow and elongated; the anchoring is distributed over the major part of the height and width of the bead. The passage of the carcass into the bead is generally substantially central relative to the walls of said bead. Furthermore, when it is a relatively narrow bead subject to predominantly axial stresses, neither the inflation pressure nor the tension induced in the carcass permits generation of large moments or torques, which tend to make the bead pivot or turn on itself.
With such a type of tyre, if the pressure drops and the vehicle continues to travel, retention of the tyre on the rim is no longer ensured, and in the majority of cases unwedging occurs.
EP 0 673 324 describes a rolling assembly comprising at least one tyre with a radial carcass reinforcement which is anchored within each bead and a rim of specific shaping. This rim comprises a first seat with a generatrix such that the axially outer end of said generatrix is distant from the axis of rotation by a length less than the distance between its axially inner end and is defined axially to the outside by a protrusion or rim flange. The tyre comprises bead seats suitable for fitting on this rim. The type of tyre/rim interface proposed in this document has many advantages compared with the solutions already known, in particular with regard to the ease of fitting/removal, while making it possible to travel a certain distance despite a drop in pressure.
EP 0 748 287 describes a solution which permits initial optimisation of the basic technology described in EP 0 673 324 referred to above. This is a tyre, at least one bead of which has a structure which makes it possible to modify the clamping of said bead according to the tension of the carcass reinforcement and in particular reinforcement thereof when the inflation pressure increases to its rated value. The document thus proposes using a bead with anchoring of the end of the carcass by turning it up about the base of the bead wire, via the axially and radially inner sides relative to the bead wire. The bead also comprises, adjacent to the bead wire and axially to the outside thereof, a profiled element of rubber compound of relatively high hardness against which the bead wire can exert a compressive force when the tension of the carcass reinforcement increases. This compressive force creates self-clamping of the toe of the bead on the fitting rim. The tension of the carcass therefore involves displacement of the bead wire towards the outside, so that the latter generates said compressive force. In such a configuration, the presence of a bead wire of conventional type and the turning-up of the carcass beneath the latter are presented as being indispensable for generating the compressive force. This restricts the other types of arrangement which can be considered
Moreover, EP 0 922 592 describes two embodiments with the carcass anchored by turning it up axially towards the outside. The first embodiment proposes anchoring of the carcass in the bead by turning it up radially towards the outside of the end of the carcass. The upturn is surrounded on either side by two radially superposed layers of metal wires arranged axially side by side and covering substantially all the axial portion along the seat of the bead. The layers are arranged so as to be parallel to the seat. The types of cords and the corresponding dimensions are very precise. The second solution proposed in this document relates to bead seats with different diameters. Securing of the carcass is also effected differently from the first solution. First of all, the carcass is subdivided into two portions which are radially separated at the level of the bead. Each portion is adjoined by a layer of cords which is arranged radially, each layer being arranged radially to the outside against each of the carcass portions. The radially outer carcass portion and the layer of cords radially to the inside are separated by an insert of the type of elastomer of high hardness provided in the bead. This insert axially lines the central portion of the bead and rises radially towards the outside and axially towards the inside, beyond the radial limit of the presence of the metal wires.
The two examples of solutions of EP 0 922 592 have several disadvantages. Thus, the securing of the carcass proposed in this document requires the presence of an upturn axially towards the outside of the end portion of the carcass. Furthermore, the superposed layers of cords are arranged radially close to the seat of the bead, for a good part at a radial position closer to the axis of rotation than the upper portion of the flange on which the bead bears. Unless highly extensible cords are used, it is difficult to fit/remove the tyre, due to the unfavourable radial position of the cords. It will also be noted that the stacks are oriented substantially parallel to the profile of the seat against which the bead bears. According to the second solution, the carcass is subdivided into two portions and an insert of high hardness is necessary to separate on one hand the layers of cords and on the other hand the two carcass portions. However, the carcass is not anchored in the insert. The form of the insert described is limitative.
Document WO 01/39999 describes a tyre with extended mobility, each of the beads of which comprises an inverted seat, an anchoring zone, a bearing zone and a transition zone. Each of the zones taken in isolation and also all the zones together to some extent form an internal bead capable of effecting relative movements, such as, for example, of the angular or rotational type, relative to another zone, or relative to a virtual centre of pressure CP, or relative to the seat of the rim, etc.
Preferably, said bearing zone is substantially elongated. It is extended, for example, substantially along the seat of the bead. The transfer of forces upon rotation of the bottom zone of the axially inner portion towards the axially outer portion is thus possible, while maintaining bearing pressure against at least a portion of the seat of the bead. The transfer of forces creates self-clamping of the toe of the bead against the rim.
As a general rule, tyres comprise architectural elements which are of symmetrical design, i.e. similar on the two sidewalls. This type of design is so widespread that it has become natural for the person skilled in the art to design tyres in accordance with such principles. However, directly applying such principles to intrinsically asymmetrical products may result in certain technical limitations or difficulties. For example, in the case of tyres provided with sidewalls of unequal heights and inverted bead seats, the stresses generated at the level of each of the beads may vary considerably. Thus, if the external sidewall is relatively tall relative to the internal sidewall of the wheel, substantially higher stresses must be withstood. Furthermore, the bead and sidewall on the exterior side of the vehicle are more exposed to the hazards of the road and the various stresses and strains which may result. These various stresses often require designers to provide multiple reinforcement and/or anchoring and/or protective elements so that the demanding conditions associated with positioning on the exterior side of the wheel can effectively be withstood. The resulting architectural elements have a significant impact in terms of weight, cost, space, difficulty and/or length of manufacture, additional materials etc. Duplicating the architecture of the external bead, said architecture being established as a function of the limit stresses for this side of the wheel, in order to obtain a bead similar on the interior side, correspondingly amplifies these various consequences.
Furthermore, many products comprise an external bead which is larger in size or surface area than the internal bead. For example, the external bead may require zones to provide protection from the effects of the external environment. In those cases in which the surface area occupied by the interior side bead is smaller than that on the exterior side, directly transposing an architecture which has been established as a function of an external bead may prove difficult or even impossible due to lack of space in the internal bead.